Items that are shipped by air typically are loaded first onto specially configured pallets or into specially configured containers. In the airfreight industry, these various pallets and containers commonly are referred to as Unit Load Devices (“ULDs”). ULDs are available in various sizes, shapes and capacities.
A ULD typically is loaded with cargo at a location other than the immediate vicinity of an aircraft. Once a ULD is loaded with cargo items, the ULD is weighed, transferred to the aircraft, and is loaded onto an aircraft through a doorway or hatch using a conveyor ramp, scissor lift, or the like. Once inside the aircraft, a ULD is moved within the cargo compartment o its final stowage position. Multiple ULDs are brought onboard the aircraft, and each is placed in its respective stowed position. Once the aircraft reaches its destination, the ULDs are unloaded from the aircraft in a manner that is the reverse of the loading procedure.
To facilitate movement of a ULD within an aircraft cargo compartment as the ULD is loaded, stowed, and unloaded, the deck of an aircraft cargo compartment typically includes a number of raised roller elements. These roller elements often include elongated roller trays that extend longitudinally along the length of the cargo deck, ball transfer units, and the like. For example, roller trays typically include elongated rows of cylindrical rollers that extend in a fore and aft direction. Ball transfer units include plates with upwardly protruding spherical balls. The ULDs sit atop these roller elements, and the roller elements facilitate rolling movement of the ULDs within the cargo compartment. Cargo decks also commonly are equipped with a plurality of power drive units (PDUs). PDUs are electrically powered rollers that can be selectively energized to propel or drive a ULD in a desired direction over a cargo deck's roller elements.
Generally, PDUs can be one of two basic types. A first type of PDU is secured to a cargo deck structure or cargo system such that the rotating axis of the powered drive roller is fixed, and the drive roller can only rotate in two opposed directions within a cargo hold. Such a “fixed” PDU typically is installed within a cargo roller tray, a ball panel, or another aircraft structure such that the PDU's drive roller protrudes above a plane defined by the uppermost portions of adjacent roller elements when the drive roller is in an active position. The drive roller can be either an inflated tire or a rigid roller having a rubber or polymer rim. The rotating tire or roller contacts and grips the bottom of an overlying ULD such that the ULD is driven in a desired direction by traction between the roller and the underside of the ULD. Such stationary PDUs often are configured such that the drive roller can be selectively moved between an active raised position, and a retracted inactive or stowed position. The lifting of the drive roller from the retracted position can be actuated by self-lifting springs, by an electrically powered lift mechanism, or the like. Such fixed PDU's typically are installed at cargo deck locations where a ULD's movement is substantially limited to two opposed directions.
A second type of PDU is known as a “steerable PDU”. In a typical steerable PDU, the drive roller is mounted to a rotatable frame or turntable that can be selectively oriented to align the drive roller in a desired direction within a cargo hold. Like the fixed PDUs described above, a steerable PDU can be configured to lift and retract the drive roller between its active raised position and its inactive retracted position. Steerable PDUs usually are installed at cargo deck locations that are proximate to an aircraft's side cargo door, where a ULD may require movement in a direction other than the fore or aft directions as the ULD is being loaded and/or unloaded.
One type of known lift mechanism 10 used in a fixed retractable PDU 60 is schematically shown in FIGS. 1A and 1B. As shown in FIG. 1A, the PDU 60 includes a rigid housing 16 and drive rollers 40. The drive rollers 40 are rotatably mounted in one end of the housing 16, and are driven by a motor disposed within the housing 16 (not shown in FIG. 1A). The opposite end of the housing 16 is pivotally mounted to an aircraft structure by hinge pins 42 that outwardly extend from the sides of the housing 16. In the PDU 60 shown in FIGS. 1A and 1B, the lift mechanism 10 includes a lift roller 30 on each side of the housing. As described in detail below, the lift rollers 30 are rotatably mounted on each end of an eccentric shaft 12. In the retracted position indicated by solid lines in FIG. 1A, each of the lift rollers 30 rests upon a top surface of a stationary reaction plate 70. In this position, the lift rollers 30 support the housing 16 and drive rollers 40 such that the tops of the drive rollers 40 are below the cargo plane 80. When the eccentric shaft 12 is rotated ninety degrees, the lift rollers 30 move downward with respect to the housing 16 and the drive rollers 40, thereby lifting the free end of the housing 16 and the drive rollers 40 to the lifted/active position shown in dashed lines in FIG. 1A. In this lifted/active position, the tops of the drive rollers 40 are above the cargo plane 80.
Details of the lift mechanism 10 are shown in FIG. 1B, which shows the mechanism 10 in a retracted position on the left side of the figure, and shows the mechanism 10 in the raised position. Each end of the shaft 12 outwardly extends from a side of the housing 16, and includes an offset roller spindle 20. As shown in FIG. 1B, each roller spindle 20 has a central axis 24 that is offset from the longitudinal axis 11 of the body of shaft 12 by a distance “a”. Circular lift rollers 30 are rotatably mounted on the spindles 20, and can include bearings 32. The lift rollers 30 have spindle receiving openings 34 at their centers, and each has an outer circumference 38 with a radius “r”.
In the retracted position shown on the left side of FIG. 1B, the shaft 12 is oriented rotationally such that the offset roller spindles 20 and lift rollers 30 are at an upward-most position relative to the housing 16. The lift rollers 30 sit atop the reaction plate 70, thereby supporting the movable end of the housing 16 at a lowermost position. Accordingly, the drive rollers 40 also are at a lowermost position, and the top surfaces of the rollers 40 are substantially below the cargo plane 80.
In the raised position shown on the right side of FIG. 1B, the shaft 12 is rotated such that the offset roller spindles 20 and lift rollers 30 move toward a lowermost position relative to the housing 16. As the shaft rotates, the lift rollers 30 bear upon the reaction plate 70, thus pushing the movable end of the housing 16 and the drive rollers 40 toward their highest position. Once the shaft 12 has rotated 180 degrees from the lowered position, the top surfaces of the rollers 40 are at or slightly above the cargo plane 80. Accordingly, the drive rollers 40 can be selectively raised and lowered by selectively rotating the shaft 12 between the raised and retracted positions with an electrical motor or other actuator (not shown in the Figs.). As indicated in FIG. 1B, the top surfaces of the drive rollers 40 are lifted a distance “H” by the lift rollers 30. The lift height “H” is a function of the degree of offset “a” between the axes 24 of the roller spindles 20 and the longitudinal axis 11 of the shaft 12.
Though the lift mechanism 10 described above is effective to selectively raise and lower the drive rollers 40, the lift mechanism 10 can have at least one shortcoming. In order to provide a sufficiently large lift height “H”, the roller spindle offset distance “a” also must be sufficiently large. Unfortunately, as the roller spindle offset distance “a” increases, the diameter “D” of the body of the shaft 12 also increases, thus also increasing the shaft's weight. The weight of a PDU's shaft 12 substantially contributes to the total weight of the PDU. Because substantial numbers of retractable PDUs often are permanently installed in cargo aircraft, and because total aircraft weight should be minimized, the total weight of each retractable PDU also should be minimized. Accordingly, a desirable property of a retractable PDU is a relatively low total weight, and more particularly, a relatively low lift-shaft weight. Therefore, it is desirable to minimize the weight of a PDU like that shown in FIGS. 1A and 1B by minimizing the diameter and weight of the shaft 12. In addition, because the space available for a PDU on an aircraft is limited, another desirable property of a retractable PDU is a relatively compact size. Therefore, it also is desirable to minimize the diameter of the shaft 12 in order to minimize the overall size of the PDU 60.
Thus there is a need for a relatively lightweight and compact retractable PDU, and more particularly, a need for a retractable PDU having a lift system that includes a shaft having a minimal diameter and a minimal weight.